Asbestos has conventionally been used as principal reinforcements for friction materials after being shaped and cured together with thermosetting resins and auxiliary reinforcements. However, asbestos, which is a carcinogenic substance presents safety and health problems during both manufacture and use. Under these circumstances, demand has increased for non-asbestos friction materials. Friction materials generally are required to have friction coefficients of from 0.3 to 0.5 and to ensure high wear resistance and mechanical strength at temperatures of about 500.degree. C.
A variety of fibrous materials have been used as non-asbestos materials. For example, Japanese Pat. Application (OPI) No. 113641/1983 (U.S. Pat. No. 4,259,397) discloses the use of preoxidized fibers produced by heat-treating acrylic (PAN) fibers at from about 200.degree. to 400.degree. C. in air (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, the preoxidized fibers with a tensile strength of from about 1 to 3 g/d and a tensile modulus of elasticity of from about 50 to 150 g/d are not satisfactory as fibrous reinforcement materials. In particular, the fibers are appreciably low in tensile modulus of elasticity as compared with asbestos. In addition, the weight of the fibers reduces at about 500.degree. C. which is the temperature that should be withstood by friction materials, and the fibers have a tendency to form cracks in the surface of the friction material due to effluent gas and shrinking in volume of the fibers.
The use of carbon fibers derived from PAN, as fibrous reinforcement materials has also been disclosed, for example, in Japanese Patent Application (OPI) Nos. 9660/80 and 9659/80 (U.S. Pat. No. 4,259,397). The carbon fibers are usually produced by calcining preoxidized fiber at 1,000.degree. C. or higher in an inert gas to obtain fibers having a carbon content of at least 90 wt % and a bond nitrogen content of from about 1 to 8 wt %, and they typically exhibit a tensile strength of at least about 13 g/d, a tensile modulus of elasticity of from about 1,400 to 1,800 g/d and an electric specific resistance of no more than about 0.01 .OMEGA.cm. However, when they are used as reinforcement for friction materials, the carbon fibers provide low friction coefficients, which is disadvantageous in the properties required in braking and power transmission. The carbon fibers also have the disadvantage that their high heat conductivity causes increased heat transfer to the support of a friction material or the mating member (usually made of a metal), thereby impairing the mechanical characteristics of the mating member.
Recently, an attempt has been made to use carbon fibers derived from pitch as fibrous reinforcement materials. These carbon fibers have a tensile strength of from about 2 to 6 g/d and a tensile modulus of elasticity of from about 200 to 300 g/d. In addition to their low performance and poor characteristics as reinforcement materials, these carbon fibers suffer from the disadvantage of low adhesion to molding resins.